The food industry is very interested in questions
like this. All organisms have certain ranges where
they do well, survive (but may not grow well or
reproduce), or do not survive. Different
bacteria have different pH zones that are optimal
(best), survivable, or deadly. There are a lot of
acidic foods, like pickles and yogurt, that are
fermented by bacteria that do well in low pH
(acidic) environments. Even with these bacteria,
there’s a lower pH limit.

The reason is a bit complicated, so let’s look
at what we mean by pH. Plain water has a pH of
7 (neutral) because it has an equal number of
hydrogen ions (protons without electrons) and
hydroxyls (OH). Hydrogen ions have a positive
charge and OH groups have a negative charge.
When there are equal numbers, they cancel each
other out. When something has a low pH, it has
a lot more hydrogen ions.

Normally, bacteria can keep their inside pH at a
constant rate, even when the environmental pH
changes. Maintaining a fairly constant environment
is called “homeostasis.” It’s a
characteristic of life. For example, our internal
body temperature stays about the same whether it’s
hot or cold out. But if you try to spend a day
outside in the summer in Death Valley or without
shelter in Antarctica, your body may not be able
to maintain homeostasis.

Back to bacteria. The outer surface and inner
membrane of some bacteria are good at keeping
protons out, but there are limits. If bacteria
that do fine in yogurt, for example, get exposed
to an even lower pH, the hydrogen ions outside get
into the inside of the cell and start to cause
trouble.

Proteins are long chains of different amino
acids. The functions of proteins usually depend on
their shapes. A protein’s shape is determined
by charges along the protein that folds up into a
particular shape. Opposite charges attract
each other. The shape is also determined by the
environment of the protein as it folds because
charges on the protein interact with water and
charged particles like hydrogen ions. A change
in pH can make proteins fold up wrong, so they
don’t do their jobs. One critical category of
protein is the enzymes, which catalyze biological
reactions. In other words, they speed up the
reactions that living things require. Enzymes
have to be a particular shape or they don’t
work. Low pH can also directly damage the DNA
or the cell membrane. Again, the hydrogen ions can
interfere with other charges, changing their
shapes and function.

Why might a food company add acids to
particular foods?
Thanks for asking,

Answer 2:

Low pH levels can change the structure and
solubility of certain proteins, small molecules,
or other substances that are essential to their
survival. For instance, if low pH values change
the structure of a protein, then the protein can
no longer perform its intended function. If
this protein is needed for starting a process that
either makes more proteins or regulates bacterial
metabolism, then the downstream effects can
quickly become catastrophic. Sometimes, the
changing of a protein's function can be as simple
as taking away, or in the case of low pH, adding a
proton onto the binding or active site of the
protein, thus changing its efficacy for
interacting with other molecules and catalyzing
reactions. Furthermore, bacteria grow most
optimally in certain ranges of pH gradients across
their membranes, so overly acidic environments,
especially in the cytoplasm of bacteria, disrupt
the pH gradient and the regulated processes of
energy transfer (in terms of the proton motive
force) in the bacterial cell. Without the function
of proteins and the maintenance of the pH
gradient, bacterial cells will quickly succumb to
cellular damage due to inhibited transcriptional,
metabolic, and repair processes and die due to the
accumulated damage.

Answer 3:

Bacteria, like all living things, use enzymes,
proteins that carry out essential functions that
keep them alive. Proteins work in a set range of
pH levels and get "denatured" - i.e. turned into
another type of molecule - if placed in a solution
with a pH outside of that range. Low pH levels
will thus denature the enzymes of any bacterium
that has not evolved to live in low-pH
environments. Since these enzymes do
essential things, denaturing them will kill the
bacterium. It would kill you, too, to dump you
into a big vat of acid, for the exact same reason,
as it would denature your enzymes and other
proteins.

Answer 4:

Low pH levels are acid pH levels, and too much
acid is toxic. pH levels lower than 7 are acidic,
and lower pH levels are more acidic.